An electrochemical system is a system that either derives electrical energy from chemical reactions, or facilitates chemical reactions through the introduction of electrical energy. An electrochemical system generally includes a cathode, an anode, and an electrolyte, and is typically complex with multiple heterogeneous subsystems, multiple scales from nanometers to meters. Examples of these systems include batteries and fuel cells. On-line characterization of batteries or fuel cells in vehicles is difficult, due to very rough noisy environments.
On-line characterization of such electrochemical systems is desirable in many applications, which include real-time evaluation of in-flight batteries on a satellite or aviation vehicle, and dynamic diagnostics of traction batteries for electric and hybrid-electric vehicles. In many battery-powered systems, the efficiency of batteries can be greatly enhanced by intelligent management of the electrochemical energy storage system. Management is only possible with proper diagnosis of the battery states.
In many battery-powered systems such as electric vehicles and satellites, real-time characterization of battery thermodynamic potential and kinetics is desirable. The characterization is crucial for battery states estimation including the state of charge (SOC), the charge and the discharge power capabilities (state of power, SOP), and the battery state of health (SOH).
A three-electrode battery structure (i.e., a battery structure that includes a reference electrode) has one more reference electrode than a conventional battery configuration, which has only two electrodes, i.e., a cathode and an anode. Due to this additional electrode, more current and voltage information is measurable than in conventional batteries. Therefore, a three-electrode configuration is very useful for diagnostics.
Although there may be many kinds of characterization models for an electrochemical system, equivalent circuit models are most appropriate in many applications where stringent real-time requirements and limiting computing powers need to be considered. In a circuit model, major effects of thermodynamic and kinetic processes in the electrochemical system can be represented by circuit elements. For example, the electrode potential between the cathode and the anode of a system can be represented with a voltage source, the charge-transfer processes can be represented with charge-transfer resistances, the double-layer adsorption can be represented with capacitances, and mass-transfer or diffusion effects can be represented with resistances such as Warburg resistances. Therefore a circuit model is extremely useful for many on-line diagnostics of the real-time states of an electrochemical system.
Improved algorithms for characterizing electrochemical systems are needed. These algorithms, and the apparatus and systems to implement them, preferably are able to broadly accept various exciting signals, are stable and robust against noises, and are agile for real-time use.
Typical in-lab experiments on three-electrode batteries are conducted around equilibrium states; therefore, the measured anode (or cathode) potential against the reference electrode is the open-circuit potential (OCV), also called thermodynamic potential, of the anode (or cathode). However, so far there hasn't been a reliable instrumentation and method to characterize each individual electrode of the battery when the battery is cycling away from equilibrium states, under a random driving profile. In many applications, such as electric vehicles, batteries are usually driven in high rates and therefore are not around equilibrium.
What is desired is a method, system, and apparatus capable of characterizing each individual electrode of a three-electrode battery, including open-circuit potentials, when the battery is cycling in a non-equilibrium state and under a random driving profile. It would be useful to estimate each individual electrode's kinetics and its change over time, to characterize battery aging analysis with impedances. It would further be useful to estimate each individual electrode's thermodynamic potential and its change over time, to characterize battery capacity and its fade over time.